Paired with a sealed cab woofer, it's a very potent LF processor. The technology is currently branded INFRA by BagEnd, but is the same as ELF (Extended Low Frequency). Check out the resources Keith suggests. You'll find the ELF design was codeveloped by none other than Ron Wickersham. There is a curious lack of user manual info on ELF/INFRA. However, BagEnd tech staff more than compensated by taking considerable phone time to explain the details and recommend approaches I could use.
I've used an ELF 118 (with processor) in my bass rig for a couple of years. It is strictly a sub (only up to about 110 Hz), but is capable of subsonic lows on the bottom end. I have it paired with a 212, which I run full range. The ELF processors are designed principally for PA use and include the crossover capability Keith mentioned. In my rig, I use the processor only to drive the sub, rather than as a crossover. The ELF processor needs its own power amp channel, but the speakers are fairly efficient (the 118 need 450 to 500w to operate comfortably).
As subs go in a bass rig, this is a different animal. The low end it delivers is very uncolored and clear. I set the 212 up on a channel and get it EQ'd, then bring the ELF sub up on its channel to give me the perfect dose of ultra lows. It winds up sounding a bit like very good movie theater sound. Definitely a good match for an Alembic.
ELF Processors generally go for $3-600 used. I picked the following brief review off the net:
http://www.bagend.com/elfm2eqreview.htm. It contains some description.
